Diffusing coating

ABSTRACT

The invention relates to the field of light boxes. It relates more specifically to a diffusing layer intended to be deposited on a substrate for making a light source homogeneous.  
     The layer according to the invention consists of agglomerated particles in a binder and is characterized by the particles having a mean diameter of between 0.3 and 2 microns, the binder being in a proportion of between 10 and 40% by volume, the particles forming aggregates whose size is between 0.5 and 20 microns, and preferably less than 5 microns, and the layer having a contrast attenuation of greater than 40%.  
     The invention also relates to uses for the production of flat lamps.

[0001] The invention relates to the field of light boxes and moreparticularly to a diffusing layer intended to be deposited on asubstrate in order to make the light source homogeneous.

[0002] Although the invention is not limited to such applications, itwill be more particularly described with reference to layers used tomake the light emitted from a light box, in particular a flat lamphomogeneous. Such a flat lamp may especially be a back-light source,used especially in flat-screen computers to illuminate a liquid-crystalscreen. They may also be architectural flat lamps used, for example, onceilings, floors or walls. They may also be flat lamps for municipalapplications, such as lamps for advertising panels or lamps able toconstitute the shelves or backs of display cabinets.

[0003] These flat lamps may also find applications in other fields, suchas, for example, the automobile industry since it is conceivable toproduce motor-vehicle roofs part of which includes such a lamp,particularly to substitute for the currently known illumination of thepassenger compartment of a motor vehicle. It is also possible to producethe backlighting for motor-vehicle dashboards.

[0004] Moreover, the expression “flat lamp” should be understood ascorresponding to a construction made from two substrates which areinitially substantially flat but which, however, may have a slightcurvature for a given application.

[0005] These flat lamps, as described for example in the U.S. Pat. No.6,034,470, therefore consist of two substantially flat substrates, suchas glass sheets, on which various layers making up the lamp aredeposited. For example, on the first glass sheet, which is the rearsheet of the lamp, silver electrodes coated with a dielectric aredeposited on the internal face and layers of alumina and phosphor aredeposited on the other face. Layers of alumina and phosphor aredeposited on the internal face of the other glass sheet, the aluminalayer forming reflection regions allowing the light emitted by the saidlamp to be made homogeneous. Other materials, such as titanium oxide,may also serve as reflecting layers.

[0006] However, it is apparent that the light thus emitted, especiallyin the case of backlighting for liquid-crystal screens, is notsufficiently homogeneous and has excessively large contrasts. Solutionsto improve the homogeneity of the light from these lamps have alreadybeen realized. Treatments of the front surface of the glass sheet, suchas frosting by sand-blasting or hot-patterning the surface of the glass,or else an opaline coloration through the thickness of the glass, havebeen proposed, among other suggestions, but are not sufficient and oftentoo expensive. A satisfactory solution from the standpoint ofhomogeneity consists in covering the front face of the glass sheet witha plastic such as a frosted polycarbonate or acrylic polymer. However,this solution has the drawback of requiring several layers of plasticcoating which result in an overall thickness of at least 5 mm. Such acoating thickness added to the other components making up the screenresults in a considerable increase in the overall thickness of the lamp.This goes counter to the current trend towards reducing the overall sizeof screens in terms of thickness. The increase in the thickness alsocauses a reduction in the luminance of the lamp. Another drawback ofsuch a lamp is that the plastic coating does not withstand the hightemperatures required for its production, especially when carrying outthe steps of depositing the electrodes and of sealing the periphery ofthe glass sheets.

[0007] The inventors were thus tasked with finding a means resulting inhomogeneity of the light emitted by a flat lamp which is at leastequivalent to the abovementioned solution but does not have itsdrawbacks, especially in terms of overall size and loss of luminance.

[0008] This objective is achieved according to the invention by adiffusing layer consisting of agglomerated particles in a binder, thesaid particles having a mean diameter of between 0.3 and 2 microns, thesaid binder being in a proportion of between 10 and 40% by volume andthe particles forming aggregates whose size is between 0.5 and 20microns and preferably less than 5 microns, the said layer having acontrast attenuation greater than 40% and preferably greater than 50%.

[0009] The contrast attenuation is determined by measuring the contrastof a test pattern; a transparent test pattern consisting of black lines8 mm in width and spaced apart by 8 mm is placed on a light table. Theupper face of the diffusing layer to be measured is placed at a distanceof 3 mm from the test pattern and an image is obtained by a camera andanalysed. The contrast attenuation is defined by c, such that:

c=1−(C/C₀)

[0010] where C is equal to the standard deviation of L over the mean ofL and C₀ is the standard deviation of L₀ over the mean of L₀, L₀ beingthe image of the test pattern without the diffuser and L being the imageof the test pattern with the diffuser present.

[0011] Such a diffusing layer can be used instead of the abovementionedplastic layers, with a markedly smaller thickness for a givenhomogeneity of the light from a given lamp.

[0012] According to a preferred embodiment of the invention, the layerhas a light transmission T_(L) greater than 45% and preferably greaterthan 60%. The light transmission is measured under illuminant D₆₅.

[0013] According to an advantageous embodiment of the invention, theparticles are semitransparent particles and preferably mineral particlessuch as oxide, nitride or carbide particles.

[0014] Preferably, the particles will be chosen from silicon, aluminium,zirconium, titanium and cerium oxides, or from a mixture of at least twoof these oxides.

[0015] Such particles may be obtained by any means known to thoseskilled in the art, and especially by precipitation or by pyrogenation.According to the invention, the particles have a size distribution suchthat at least 50% of the particles differ by less than 50% from the meandiameter.

[0016] According to an advantageous embodiment of the invention, thebinder has a temperature resistance sufficient to withstand theoperating temperatures and/or the sealing temperature of the lamp if thelayer is produced during assembly of the lamp and especially before thelatter is sealed. In this regard, a mineral binder proves to beparticularly advantageous when the diffusing layer must withstand a hightemperature, greater than about 300° C.

[0017] When the layer is in the external position, the binder is alsoadvantageously chosen so as to have an abrasion resistance sufficient toundergo, without any damage, all the handling operations carried out onthe lamp, for example especially when mounting the flat screen.

[0018] Depending on the requirements, the binder chosen may be a mineralbinder, for example when it is desired to obtain a layer which resistshigh temperatures, or an organic binder, especially to simplify theproduction of the said layer, crosslinking possibly being obtainedsimply, for example when cold. Choosing a mineral binder having a hightemperature resistance will, in particular, allow large flat lamps to beproduced without any risk of degradation of the layer appearing causedby, for example, fluorescent tubes which generate a considerable amountof heat. This is because, with the known solutions, thermal degradationof the plastic film occurs, which therefore makes it very difficult toproduce large flat lamps.

[0019] Advantageously, the binder has a refractive index different fromthat of the particles and the difference between these two indices ispreferably at least 0.1. The refractive index of the particles isadvantageously greater than 1.7 and that of the binder is preferablyless than 1.6.

[0020] According to a preferred embodiment of the invention, the binderis chosen from mineral binders, such as potassium silicates, sodiumsilicates, lithium silicates and aluminium phosphates, and organicbinders, such as polymers of the polyvinyl alcohol, thermosetting resinand acrylic types. A mineral binder is particularly preferred.

[0021] To encourage the formation of aggregates with the desired size,the invention advantageously provides for the addition of at least oneadditive resulting in a random distribution of the particles in thebinder. Preferably, the dispersing additive or agent is chosen from thefollowing agents: an acid, a base, divalent ions and ionic polymers oflow molecular mass, especially less than 50 000 g/mol.

[0022] It is also possible to add other agents, for example a wettingagent such as nonionic, anionic or cationic surfactants, in order toprovide a layer which is homogeneous on a large scale. It is alsopossible to add rheology modifiers, such as cellulose ethers.

[0023] The layer thus defined may be deposited with a thickness ofbetween 1 and 20 microns. The methods of depositing such a layer may beall means known to those skilled in the art, such as depositing byscreen printing, by coating a paint, by dip coating, by spin coating, byflow coating, by spraying, etc.

[0024] When the desired thickness of the deposited layer is greater than2 microns, it is advantageous to use a deposition process of thescreen-printing type.

[0025] When the thickness of the layer is less than 4 microns, it ispreferably deposited by flow coating or by spraying.

[0026] The invention also provides for the production of a layer whosethickness varies according to the area of coverage on the surface; suchan embodiment may allow intrinsic inhomogeneities in a light source tobe corrected. For example, it is possible in this way to correct thevariation in illumination of a fluorescent tube along its length.According to another embodiment, resulting substantially in the sameeffect of correcting the intrinsic inhomogeneities of a light source,the invention provides for there to be a layer whose covering densityvaries over the deposition surface; this may, for example, be a coating,deposited by screen printing, the density of spots of which varies froma completely covered region to a region of dispersed spots, thetransition being gradual or otherwise.

[0027] According to the invention, such layers may be deposited ontransparent or semitransparent substrates which, depending on theapplication, have a planar shape or a non-planar shape. The applicationsenvisaged by the invention are especially flat lamps, for example thoseused for the illumination of liquid-crystal screens, or forarchitectural illumination or else for municipal illumination.

[0028] In the case of flat lamps, the layer is advantageously depositedon the glass sheet constituting the front face of the lamp.

[0029] According to a first embodiment, the layer is deposited on thatface of the glass sheet which is on the inside of the lamp; according tosuch an embodiment, the layer must be deposited on the glass sheetduring production of the lamp. According to this embodiment, the layermust have a temperature resistance sufficient to withstand the variousheat treatments needed to produce such a lamp, especially for carryingout the deposition operations corresponding to the production of theelectrodes and for sealing the periphery of the two glass sheets formingthe structure of the flat lamp. The layer according to the inventionwill therefore be advantageously made with a mineral binder.

[0030] According to this first embodiment, the diffusing layer accordingto the invention may, according to one variant, be deposited directly onthe glass or, according to a second embodiment, be deposited on thelayers that have already been deposited on the glass. If spacers arenecessary, especially for keeping a uniform space between the two glasssheets, the invention advantageously provides for deposition of thediffusing layer leaving free regions corresponding to the locationsintended for the spacers, in such a way that the adhesion of the spacersis not disturbed by the layer according to the invention. Such freespaces may easily be obtained by choosing to deposit the layer using ascreen-printing technique.

[0031] According to a second embodiment, the layer is deposited on thatface of the glass sheet which is on the outside of the lamp; accordingto this embodiment, the diffusing layer according to the invention isadvantageously chosen to have enhanced mechanical-strength propertiesand more particularly abrasion-resistance properties. The layer is thenadvantageously produced with a mineral or organic binder in an amountgreater than 15% and preferably greater than 20%.

[0032] According to an alternative embodiment of the invention, relatingto the use of the diffusing layer in the production of a flat lamp, thesaid diffusing layer is deposited on a transparent or semitransparentsubstrate independent of the glass sheets forming the structure of theflat lamp. Such an embodiment may consist in depositing the diffusinglayer on a glass substrate held at a certain distance from the frontface of the lamp; this embodiment makes it possible, according to thelaws of physics, to further improve the diffusing effect of the layer.This is offset by the fact that the volume or overall size of such aconstruction becomes equivalent to the previously known solutions, butin this case with better performance as regards diffusion.

[0033] The invention also advantageously provides for a diffusing layerto be deposited on each side of the substrate, whether this is one ofthe glass sheets forming the structure of the flat lamp or anindependent substrate.

[0034] The diffusing layers thus presented according to the inventiontherefore make it possible to produce flat lamps intended, for example,for the illumination of liquid-crystal screens. Compared with thepreviously known solutions, the layer according to the invention makesit possible to reduce the overall size of the said lamp for a givenperformance in terms of contrast attenuation.

[0035] Depending on the alternative uses of these layers, theperformance in terms of contrast attenuation of the light emitted byflat lamps can be improved for an overall size of the said lamps whichis equivalent to that of the prior art.

[0036] Further details and advantageous features will become apparentbelow from the description of the illustrative examples of the inventionwhich follow.

[0037] The first example corresponds to a layer according to theinvention intended to be deposited on the front face of a flat lamp forthe illumination of a liquid-crystal screen.

[0038] The following mixture was produced:

[0039] 15 g of alumina particles having a mean diameter of 1 micron wereput into 65.5 g of deionized water, into which 0.4 g of 50% polyacrylicacid, as dispersion-inducing additive, and 0.3 g of a wettingagent—octylphenol ethoxylate sold by Union Carbide under the name TRITONX-100—were added.

[0040] The solution thus prepared was mixed in a turbine for twominutes.

[0041] Next, 19 g of a 20% aqueous solution of polyvinyl alcohol wereadded and the mixture thus formed put back through the turbine for 5minutes.

[0042] A diffusing layer was then produced from this mixture on a glasssheet by a flow-coating technique. The mixture was deposited with anamount such that the layer once dried had a thickness of 1.5 microns.

[0043] Example 2 is a layer produced according to the invention from themixture of Example 1 deposited on a glass sheet in order to form a drylayer having a thickness of 4 microns.

[0044] Example 3 is an layer produced according to the invention fromthe mixture of Example 1 deposited on a glass sheet in order to form adry layer having a thickness of 1 micron.

[0045] Example 4 is a layer according to the invention from thefollowing mixture.

[0046] 15 g of alumina particles having a mean diameter of 1 micron wereintroduced into 65.5 g of deionized water, to which 0.4 g of 50%polyacrylic acid as dispersion-inducing additive and 0.3 g of a wettingagent—octylphenol ethoxylate sold by Union Carbide under the name TRITONX-100—were added.

[0047] The solution thus prepared was mixed in a turbine for twominutes.

[0048] Next, 32 g of a 20% aqueous solution of polyvinyl alcohol wasadded and the mixture thus formed put back through the turbine for 5minutes.

[0049] A diffusing layer was then produced from this mixture on a glasssheet by a flow-coating technique. The mixture was deposited with anamount such that the layer once dried had a thickness of 4 microns.

[0050] Example 5 is a layer according to the invention from thefollowing mixture.

[0051] 15 g of alumina particles having a mean diameter of 1 micron wereintroduced into 65.5 g of deionized water, to which 0.4 g of 50%polyacrylic acid as dispersion-inducing additive and 0.3 g of a wettingagent—octylphenol ethoxylate sold by Union Carbide under the name TRITONX-100—were added.

[0052] The solution thus prepared was mixed in a turbine for twominutes.

[0053] Next, 50 g of a 20% aqueous solution of polyvinyl alcohol wereadded and the mixture thus formed put back through the turbine for fiveminutes.

[0054] A diffusing layer was then produced from this mixture on a glasssheet by a flow-coating technique. The mixture was deposited with anamount such that the layer once dried had a thickness of 4 microns.

[0055] Example 6 is a layer produced according to the invention from amixture described below, in which a mineral binder is used.

[0056] 15 g of alumina particles having a mean diameter of 1 micron wereintroduced into 72 g of deionized water, to which 0.4 g of 50%polyacrylic acid as dispersion-inducing additive were added. Next,sodium hydroxide was added until a pH of 10 was reached. The mixture wasthen homogenized in a turbine for two minutes.

[0057] Next, 12.7 g of a 25% aqueous solution of lithium silicate binderand 0.3 g of a wetting agent—octylphenol ethoxylate sold by UnionCarbide under the name TRITON X-100—were added.

[0058] The solution thus prepared was again put through a turbine for 5minutes.

[0059] A diffusing layer was then produced from this mixture on a glasssheet by a flow-coating technique. The mixture was deposited with anamount such that the layer once dried had a thickness of 2 microns.

[0060] Examples 7 and 8 are comparative examples which use a techniqueof the prior art mentioned above, which consists in using as diffusingelement frosted plastic films which are placed in front of the frontface of the flat lamp.

[0061] Example 7 corresponds to a plastic film having a thickness of 1mm.

[0062] Example 8 corresponds to the superposition of two plastic filmsidentical to that of Example 7 in order to obtain a total thickness of 2mm.

[0063] Example 9 is a layer produced according to the invention from themixture described below.

[0064] 4 g of a suspension of 50% polyacrylic acid by weight in waterand sodium hydroxide (NaOH) were added to 549 g of deionized water so asto obtain a pH of 9. Next, 288 g of an aqueous suspension containing 52%by weight of zirconia particles having a mean diameter of 1 micron (soldby Norton under the name SLURRY 9839) were added to the mixture. Themixture thus prepared was put through a turbine for 10 minutes. Next,158 g of a suspension of a 20% lithium polysilicate binder by weight inwater (sold by Grace under the name LUDOX) and 1 g of a wetting agent(sold by Union Carbide under the name TRITON X-100) were added.

[0065] A diffusing layer was then produced from this mixture on a glasssheet using a flow-coating technique. The mixture was deposited in anamount such that the layer once dried had a thickness of 2 microns.

[0066] Example 10 is a layer produced according to the invention fromthe following mixture.

[0067] 4 g of a polyacrylic acid suspension containing 50% by weight inwater and sodium hydroxide (NaOH) were added to 687 g of deionized waterso as to obtain a pH of 9. Next, 150 g of alumina particles having amean diameter of 1 μm were added to the mixture. The mixture thusprepared was put through a turbine for 10 minutes. Next, 158 g of abinder suspension containing 20% by weight lithium polysilicate in water(sold by Grace under the name LUDOX) and 1 g of a wetting agent (sold byUnion Carbide under the name TRITON X-100) were added.

[0068] Next, a diffusing layer was produced from this mixture on a glasssheet using a flow-coating technique. The mixture was deposited in anamount such that the layer once dried had a thickness of 2.5 microns.

[0069] Example 11 is a comparative example which consists in using thefollowing mixture in which the particles have a small size.

[0070] 4 g of a suspension of 50% by weight polyacrylic acid in waterand sodium hydroxide (NaOH) were added to 87 g of deionized water so asto obtain a pH of 9. Next, 750 g of a suspension of alumina particles(mean diameter 0.2 microns) containing 20% particles by weight in waterwere added. The mixture thus prepared was put through a turbine for 10minutes. Next, 158 g of a suspension containing 20% lithium polysilicatein water (sold by Grace under the name LUDOX) and 1 g of a wetting agent(sold by Union Carbide under the name TRITON X-100) were added.

[0071] Next, a diffusing layer was produced from this mixture on a glasssheet using a flow-coating technique. The mixture was deposited in anamount such that the layer once dried had a thickness of 2 microns.

[0072] The various diffusing layers according to Examples 1 to 11 thusdeposited on a glass sheet were then tested according to the contrastattenuation measurement method described above.

[0073] Light transmission measurements under illuminant D₆₅ were carriedout on these various examples.

[0074] The resistance to scratching by a fingernail of the diffusinglayers according to Examples 3 and 9 to 11 were also assessed visually.The resistance is termed “good” when the layer does not become detachedfrom the glass sheet and “poor” when the layer delaminates, revealingthe glass, after the surface has been scratched with a fingernail.

[0075] The temperature resistance of the diffusing layers was determinedafter the said layers had been subjected to 450° C. for 1 hour.Resistance denoted “−” means that the layer examined by the naked eyehad at least partially degraded. Conversely, the resistance is denoted“+” when the layer retains its original appearance.

[0076] The various results obtained are given in the table below.Contrast attentuation T_(L) Scratch Temperature Example (%) (%)resistance resistance 1 52 66 n.d − 2 65 47 n.d − 3 43 76 poor − 4 54 64n.d. − 5 48 69 n.d. − 6 62 60 n.d. + 7 (comparative) 43 72 n.d. − 8(comparative) 62 51 n.d. − 9 64 57 good + 10  66 55 good + 11(comparative) <20 78 good +

[0077] n.d.: not determined.

[0078] The results obtained show that the diffusing layers according tothe invention result in a contrast attenuation and therefore ahomogeneity of the light, for example in a flat lamp for aliquid-crystal screen, which is better than that using the already-knowntechniques for the same overall size and a given light transmission.

[0079] These results also show that, for the same overall size and for agiven contrast attenuation, the diffusing layers according to theinvention result in a better light transmission than that obtained usingthe prior techniques.

[0080] From these results, it is also possible to deduce that, for agiven contrast attenuation and a given light transmission, the diffusinglayers according to the invention make it possible to produce, forexample, a flat lamp having a smaller overall size than the priortechniques.

[0081] It should be noted that the layers produced from a mineral binderhave a better scratch resistance and a better temperature resistance.

1. Diffusing layer consisting of agglomerated particles in a binder,characterized in that the particles have a mean diameter of between 0.3and 2 microns, in that the binder is in a proportion of between 10 and40% by volume, in that the particles form aggregates whose size isbetween 0.5 and 20 microns, and preferably less than 5 microns, and inthat the layer has a contrast attenuation of greater than 40%. 2.Diffusing layer according to claim 1, characterized in that it has alight transmission T_(L) of greater than 45% and preferably greater than60%.
 3. Diffusing layer according to claim 1 or 2, characterized in thatthe particles are semitransparent particles and preferably mineralparticles, such as oxide, nitride or carbide particles.
 4. Diffusinglayer according to one of claims 1 to 3, characterized in that theparticles are silicon, aluminium, zirconium, titanium or cerium oxides,or a mixture of at least two of these oxides.
 5. Diffusing layeraccording to one of claims 1 to 4, characterized in that it includes adispersing additive such as an acid, a base, divalent ions or ionicpolymers of low molecular mass.
 6. Diffusing layer according to one ofclaims 1 to 5, characterized in that it has a thickness of between 1 and20 microns.
 7. Diffusing layer according to one of claims 1 to 6,characterized in that the binder is chosen from mineral binders, such aspotassium silicates, sodium silicates, lithium silicates and aluminiumphosphates, and organic binders, such as polymers of the polyvinylalcohol, thermosetting resin and acrylic types.
 8. Diffusing layeraccording to claim 7, characterized in that the binder is a mineralbinder.
 9. Use of a diffusing layer as described according to one ofclaims 1 to 8 for producing a diffusing substrate in a flat lamp. 10.Use of a diffusing layer according to claim 9, characterized in that itis deposited on a flat substrate, preferably made of glass.
 11. Use of adiffusing layer according to either of claims 9 and 10, characterized inthat the flat substrate is one of the glass sheets forming the flatlamp.
 12. Use of a diffusing layer according to either of claims 10 and11, characterized in that it is deposited on both sides of the flatsubstrate.
 13. Use of a diffusing layer according to one of claims 9 to12, characterized in that it is deposited using a deposition method suchas screen printing, coating with a paint, dip coating, spin coating,flow coating or spraying.
 14. Use of a diffusing layer according to oneof claims 9 to 13, characterized in that the thickness and/or thecovering density of the layer varies over the deposition surface.